This invention relates generally to locomotive railyard management, and more specifically, to determining the location of a locomotive in a railyard.
Railyards are the hubs of railroad transportation systems. Therefore, railyards perform many services, for example, freight origination, interchange, and termination, locomotive storage and maintenance, assembly and inspection of new trains, servicing of trains running through the facility, inspection and maintenance of railcars, and railcar storage. The various services in a railyard compete for resources such as personnel, equipment, and space in various facilities so that managing the entire railyard efficiently is a complex operation. Knowing the location of locomotives within the railyard is useful information for effective management of a railyard and railyard resources.
Satellite tracking systems, such a Global Positioning Systems (GPS), are traditionally used to track locomotive location. However, prior to May 2, 2000 the Department of Defense restricted the use of GPS limiting its accuracy. This limited accuracy mode is referred to as selective availability (SA). While SA was active, the GPS satellite signals included pseudorandom perturbations of the timing data broadcast in the signal. These perturbations caused geolocation algorithms used in GPS receivers to miscalculate the distance to each satellite causing the geometric solution for the location of the receiver to be less accurate. Additionally, GPS signals are affected by refraction and multipath reflections, associated with conditions in the ionosphere and troposphere. The obsolescence of SA removes a significant source of error in GPS signals.
The refraction and multipath effects visited on the signals arriving from GPS satellites remain a source of error. The concept of differential Global Positioning Systems (DGPS) can be used to overcome refraction and multipath effects. A traditional DGPS system includes a single fixed base station receiver and one or more other receivers, which are generally referred to as the mobile units. The base station GPS antenna is placed at a precisely known location so that each GPS-based position estimate obtained at the base station can be compared to its known position, and the error assessed. The mobile units also obtain GPS-based position estimates, which are likewise subject to error, but since they are not generally at precisely known positions, they have no better information available than that provided by the GPS estimate.
The nature of the GPS error is that the refraction and multipath effects visited on the signals arriving from the GPS satellites are nearly identical within a small area on the Earth. Thus, mobile units will be subject to nearly the same errors as the base station, so if the base station transmits that error profile to the mobile units, they will be able to correct their GPS estimates and establish much more accurate positions than otherwise.
However, if the base station and mobile unit are using different sets of satellites to compute position during the relevant interval of collection, then the positional error of the base station will be quite different from the error computed at the mobile unit. Additionally, if the mobile unit is collecting GPS readings at different moments than the base station, some small amount of error is introduced to the DGPS process.
In a railyard scenario, locomotives equipped with a mobile GPS receiver move on tracks having a minimal separation of thirteen feet. Therefore, a location tracking system that can identify the location of a locomotive with a tolerance of less than thirteen feet is beneficial in optimizing railyard management. To optimize locomotive location accuracy within a railyard, there exists a need to resolve the error in DGPS locomotive location systems caused by the base station and locomotive receiving signals from different satellites and at different times.
In one embodiment, a system is provided for overcoming errors in DGPS locomotive location systems and accurately determining a locomotive location within a railyard. The system includes a base station having a known fixed position, which includes a GPS receiver and a computer to receive data and compute locomotive location estimates, a mobile GPS receiver and a computer coupled to the locomotive, and a one-way communications link from the locomotive to the base station to transmit locomotive GPS data. The locomotive computer sends aggregated GPS data to the base station computer, in a form that allows the base station computer to obtain a refined position estimate for the locomotive, using mathematical functions.
More particularly, the location of a locomotive is tracked using data from satellites that are a subset of the satellite constellation used to track the location of a base station. A mobile receiver coupled to the locomotive communicates to a computer a stream of data containing an estimated position of the locomotive and data pertaining to the satellites used in gathering the location data and the times the data was received. A base station receiver continuously sends base station location data to the computer, which electronically stores the data. The computer interprets the information in the data stream and computes an estimated position for the base station. The computer then determines a positional error using a known fixed location of the base station and applies this positional error to the locomotive estimated position to obtain an accurate location of the locomotive, thereby improving railyard management by determining locomotive position to the correct track.